Polyketide synthase (PKS) systems are generally known in the art as enzyme complexes derived from fatty acid synthase (FAS) systems, but which are often highly modified to produce specialized products that typically show little resemblance to fatty acids. It has now been shown, however, that polyketide synthase systems exist in marine bacteria and certain microalgae that are capable of synthesizing PUFAs from malonyl-CoA. The PKS pathways for PUFA synthesis in Shewanella and another marine bacteria, Vibrio marinus, are described in detail in U.S. Pat. No. 6,140,486. The PKS pathways for PUFA synthesis in the eukaryotic Thraustochytrid, Schizochytrium is described in detail in U.S. Pat. No. 6,566,583. Finally, the PKS pathways for PUFA synthesis in eukaryotes such as members of Thraustochytriales, including the complete structural description of the PUFA PKS pathway in Schizochytrium and the identification of the PUFA PKS pathway in Thraustochytrium, including details regarding uses of these pathways, are described in detail in U.S. Patent Application Publication No. 20020194641, published Dec. 19, 2002 (corresponding to U.S. patent application Ser. No. 10/124,800, filed Apr. 16, 2002).
Researchers have attempted to exploit polyketide synthase (PKS) systems that have been described in the literature as falling into one of three basic types, typically referred to as: Type II, Type I and modular. The Type II system is characterized by separable proteins, each of which carries out a distinct enzymatic reaction. The enzymes work in concert to produce the end product and each individual enzyme of the system typically participates several times in the production of the end product. This type of system operates in a manner analogous to the fatty acid synthase (FAS) systems found in plants and bacteria. Type I PKS systems are similar to the Type II system in that the enzymes are used in an iterative fashion to produce the end product. The Type I differs from Type II in that enzymatic activities, instead of being associated with separable proteins, occur as domains of larger proteins. This system is analogous to the Type I FAS systems found in animals and fungi.
In contrast to the Type I and II systems, in modular PKS systems, each enzyme domain is used only once in the production of the end product. The domains are found in very large proteins and the product of each reaction is passed on to another domain in the PKS protein. Additionally, in all of the PKS systems described above, if a carbon-carbon double bond is incorporated into the end product, it is always in the trans configuration.
In the Type I and Type II PKS systems described above, the same set of reactions is carried out in each cycle until the end product is obtained. There is no allowance for the introduction of unique reactions during the biosynthetic procedure. The modular PKS systems require huge proteins that do not utilize the economy of iterative reactions (i.e., a distinct domain is required for each reaction). Additionally, as stated above, carbon-carbon double bonds are introduced in the trans configuration in all of the previously described PKS systems.
Polyunsaturated fatty acids (PUFAs) are critical components of membrane lipids in most eukaryotes (Lauritzen et al., Prog. Lipid Res. 40 1 (2001); McConn et al., Plant J. 15, 521 (1998)) and are precursors of certain hormones and signaling molecules (Heller et al., Drugs 55, 487 (1998); Creelman et al., Annu. Rev. Plant Physiol. Plant Mol. Biol. 48, 355 (1997)). Known pathways of PUFA synthesis involve the processing of saturated 16:0 or 18:0 fatty acids (the abbreviation X:Y indicates an acyl group containing X carbon atoms and Y double bonds (usually cis in PUFAs); double-bond positions of PUFAs are indicated relative to the methyl carbon of the fatty acid chain (ω3 or ω6) with systematic methylene interruption of the double bonds) derived from fatty acid synthase (FAS) by elongation and aerobic desaturation reactions (Sprecher, Curr. Opin. Clin. Nutr. Metab. Care 2, 135 (1999); Parker-Barnes et al., Proc. Natl. Acad. Sci. USA 97, 8284 (2000); Shanklin et al., Annu. Rev. Plant Physiol. Plant Nol. Biol. 49, 611 (1998)). Starting from acetyl-CoA, the synthesis of docosahexaenoic acid (DHA) requires approximately 30 distinct enzyme activities and nearly 70 reactions including the four repetitive steps of the fatty acid synthesis cycle. Polyketide synthases (PKSs) carry out some of the same reactions as FAS (Hopwood et al., Annu. Rev. Genet. 24, 37 (1990); Bentley et al., Annu. Rev. Microbiol. 53, 411 (1999)) and use the same small protein (or domain), acyl carrier protein (ACP), as a covalent attachment site for the growing carbon chain. However, in these enzyme systems, the complete cycle of reduction, dehydration and reduction seen in FAS is often abbreviated so that a highly derivatized carbon chain is produced, typically containing many keto- and hydroxy-groups as well as carbon-carbon double bonds in the trans configuration. The linear products of PKSs are often cyclized to form complex biochemicals that include antibiotics and many other secondary products (Hopwood et al., (1990) supra; Bentley et al., (1999), supra; Keating et al., Curr. Opin. Chem. Biol. 3, 598 (1999)).
Very long chain PUFAs such as docosahexaenoic acid (DHA; 22:6ω3) and eicosapentaenoic acid (EPA; 20:5ω3) have been reported from several species of marine bacteria, including Shewanella sp (Nichols et al., Curr. Op. Biotechnol. 10, 240 (1999); Yazawa, Lipids 31, S (1996); DeLong et al., Appl. Environ. Microbiol. 51, 730 (1986)). Analysis of a genomic fragment (cloned as plasmid pEPA) from Shewanella sp. strain SCRC2738 led to the identification of five open reading frames (Orfs), totaling 20 Kb, that are necessary and sufficient for EPA production in E. coli (Yazawa, (1996), supra). Several of the predicted protein domains were homologues of FAS enzymes, while other regions showed no homology to proteins of known function. At least 11 regions within the five Orfs were identifiable as putative enzyme domains (See Metz et al., Science 293:290-293 (2001)). When compared with sequences in the gene databases, seven of these were more strongly related to PKS proteins than to FAS proteins. Included in this group were domains putatively encoding malonyl-CoA:ACP acyltransferase (MAT), β-ketoacyl-ACP synthase (KS), β-ketoacyl-ACP reductase (KR), acyltransferase (AT), phosphopantetheine transferase, chain length (or chain initiation) factor (CLF) and a highly unusual cluster of six ACP domains (i.e., the presence of more than two clustered ACP domains had not previously been reported in PKS or FAS sequences). It is likely that the PKS pathway for PUFA synthesis that has been identified in Shewanella is widespread in marine bacteria. Genes with high homology to the Shewanella gene cluster have been identified in Photobacterium profundum (Allen et al., Appli. Environ. Microbiol. 65:1710 (1999)) and in Moritella marina (Vibrio marinus) (see U.S. Pat. No. 6,140,486, ibid., and Tanaka et al., Biotechnol. Lett. 21:939 (1999)).
Polyunsaturated fatty acids (PUFAs) are considered to be useful for nutritional, pharmaceutical, industrial, and other purposes. An expansive supply of PUFAs from natural sources and from chemical synthesis are not sufficient for commercial needs. A major current source for PUFAs is from marine fish; however, fish stocks are declining, and this may not be a sustainable resource. Additionally, contamination, both heavy metal and toxic organic molecules, is a serious issue with oil derived from marine fish. Vegetable oils derived from oil seed crops are relatively inexpensive and do not have the contamination issues associated with fish oils. However, the PUFAs found in commercially developed plant oils are typically limited to linoleic acid (eighteen carbons with 2 double bonds, in the delta 9 and 12 positions—18:2 delta 9,12) and linolenic acid (18:3 delta 9,12,15). In the conventional pathway for PUFA synthesis, medium chain-length saturated fatty acids (products of a fatty acid synthase (FAS) system) are modified by a series of elongation and desaturation reactions. Because a number of separate desaturase and elongase enzymes are required for fatty acid synthesis from linoleic and linolenic acids to produce the more saturated and longer chain PUFAs, engineering plant host cells for the expression of PUFAs such as EPA and docosahexaenoic acid (DHA) may require expression of several separate enzymes to achieve synthesis. Additionally, for production of useable quantities of such PUFAs, additional engineering efforts may be required, for example, engineering the down regulation of enzymes that compete for substrate, engineering of higher enzyme activities such as by mutagenesis or targeting of enzymes to plastid organelles. Therefore it is of interest to obtain genetic material involved in PUFA biosynthesis from species that naturally produce these fatty acids and to express the isolated material alone or in combination in a heterologous system which can be manipulated to allow production of commercial quantities of PUFAs.
The discovery of a PUFA PKS system in marine bacteria such as Shewanella and Vibrio marinus(see U.S. Pat. No. 6,140,486, ibid.) provides a resource for new methods of commercial PUFA production. However, these marine bacteria have limitations which may ultimately restrict their usefulness on a commercial level. First, although U.S. Pat. No. 6,140,486 discloses that these marine bacteria PUFA PKS systems can be used to genetically modify plants, the marine bacteria naturally live and grow in cold marine environments and the enzyme systems of these bacteria do not function well above 22° C. In contrast, many crop plants, which are attractive targets for genetic manipulation using the PUFA PKS system, have normal growth conditions at temperatures above 22° C. and ranging to higher than 40° C. Therefore, the PUFA PKS systems from these marine bacteria are not predicted to be readily adaptable to plant expression under normal growth conditions. Additionally, the known marine bacteria PUFA PKS systems do not directly produce triacylglyerols (TAG), whereas direct production of TAG would be desirable because TAG are a lipid storage product, and as a result, can be accumulated at very high levels in cells, as opposed to a “structural” lipid product (e.g. phospholipids), which can generally only accumulate at low levels.
With regard to the production of eicosapentaenoic acid (EPA) in particular, researchers have tried to produce EPA with microbes by growing them in both photosynthetic and heterotrophic cultures. They have also used both classical and directed genetic approaches in attempts to increase the productively of the organisms under culture conditions. Other researchers have attempted to produce EPA in oil-seed crop plants by introduction of genes encoding various desaturase and elongase enzymes.
Researchers have attempted to use cultures of red microalgae (Monodus), diatoms (e.g. Phaeodactylum), other microalgae and fungi (e.g. Mortierella cultivated at low temperatures). However, in all cases, productivity was low compared to existing commercial microbial production systems for other long chain PUFAs such as DHA. In many cases, the EPA occurred primarily in the phospholipids (PL) rather than the triacylglycerols (TAG). Since productivity of microalgae under heterotrophic growth conditions can be much higher than under phototrophic conditions, researchers have attempted, and achieved, trophic conversion by introduction of genes encoding specific sugar transporters. However, even with the newly acquired heterotrophic capability, productivity in terms of oil remained relatively low.
Efforts to produce EPA in oil-seed crop plants by modification of the endogenous fatty acid biosynthesis pathway have only yielded plants with very low levels of the PUFA in their oils. As discussed above, several marine bacteria have been shown to produce PUFAs (EPA as well as DHA). However, these bacteria do not produce TAG and the EPA is found primarily in the PL membranes. The levels of EPA produced as well as the growth characteristics of these particular marine bacteria (discussed above) limit their utility for commercial production of EPA.
Therefore, there is a need in the art for other PUFA PKS systems having greater flexibility for commercial use, and for a biological system that efficiently produces quantities of lipids (PL and TAG) enriched in desired PUFAs, such as EPA, in a commercially useful production process.